Nicotinic acetylcholine receptors (AChRs) at the nerve-muscle synapse enter and recover from D(esensitized) states that remain inactive in the continuous presence of neurotransmitter. Receptor desensitization down-regulates cell responses and encodes long-term activity patterns. Our goal is to understand the AChR desensitization process at a molecular level. The essential problem of how D states are connected to C(losed) and O(pen) gating states remains unsolved. We will investigate two hypotheses: i) D, O and C are connected in a closed cycle and ii) D is not connected either to C or O but rather to a short-lived gating intermediate state. We will test the second hypothesis experimentally by using mutations and voltage to shift the point of bifurcation between gating and desensitization conformational-change pathways, towards C to produce openings upon recovery or towards O to prevent openings. Many amino acids change structure during the AChR gating and desensitization transitions, and each of these microscopic rearrangements has an associated and mostly-local free energy change. We will measure these energy changes in desensitization at two regions in the AChR transmembrane domain: i) a proline kink in M1 and ii) the hydrophobic M2 gate. Despite intensive investigation, the molecular basis of AChR desensitization has remained obscure since the process was first described 60 years ago. The experiments in this proposal will fill this substantial gap in our understanding of receptor operation.